The detection of sexual orientation by outward appearance or behavior is frequently challenged by situations in which masculine gay men who do not act in a stereotypically "gay" fashion, or with metrosexual men (regardless of sexuality) who exhibit a lifestyle, spending habits, and concern for personal appearance stereotypical of fashionable urban gay men.[1][2][3][4]

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Most of the research on this issue can be filed into the field called physiognomy, very popular in the 19th century when it has been used as a basis for scientific racism, along with physical anthropology.[5]

A number of scientific studies have been conducted to test whether gaydar is real or just a popular myth. Perhaps the earliest study[6] asked people to judge sexual orientation from video clips, with results concluding that it was a myth. A later study in the Journal of Personality and Social Psychology showed that people could judge sexual orientation more accurately than chance.[7] This study asked people to indicate their sexual orientation using the Kinsey scale and then had others view very brief silent clips of the people talking using thin-slicing. The viewers rated their sexual orientations on the same scale and the researchers found a significant correlation between where the people said they were on the scale and where they were perceived to be on the scale. Later studies have repeated this finding[8] and have even shown that home videos of children can be used to judge accurately their sexual orientation later in life.[9]

Later studies found that gaydar was also accurate at rates greater than chance for judgments just from the face. Study participants use gendered facial cues and stereotypes of gay people to make their judgments, but reliably misjudge sexual orientation for people countering stereotypes.[10] The race, ethnicity, and nationality of neither the person making the judgment nor the person they are judging seems to make a difference when making judgments from faces.[11][12][13] Even individual facial features (just the eyes) can sometimes give enough information to tell whether a man or woman is gay, straight, or lesbian.[14][15] One study showed that judgments of men's[16] and women's[15] faces for about 1/25 of a second was enough time to tell whether they were gay, straight, or lesbian. People's judgments were no more accurate when they had more time to make their judgments. Follow-up work to this suggested that gaydar happens automatically when someone sees another person and that seeing someone’s face automatically activates stereotypes about gays and straights.[17] People seem not to know that they have gaydar, though.[14] Gay men have more accurate gaydar than straight men,[18] and women have more accurate gaydar when they are ovulating.[19] One study hypothesized that this might be because homosexual people are more attentive to detail than heterosexual people are, apparently as an adopted perceptual style aiding in the recognition of other homosexual people.[20]

Other studies have found that men and women with body shapes and walking styles similar to people of the opposite sex are more often perceived as gay.
The study, by UCLA assistant professor Kerri Johnson, found that observers were able to accurately guess the sexual orientation of men 60 percent of the time — almost a coin toss; with women, their guesses didn't exceed chance. But what's most interesting to researchers is understanding how that snap judgment can unleash a series of stereotypes. Contrary to hype surrounding the study, the results suggest that walking styles and body shapes do not give away sexual orientation. The study was intended to reveal information about the perception of the observer, but has been misinterpreted as conveying reliable information about the sexual orientation of the participants. Gender-specific body movements are not reliably associated with a person's sexual orientation;[21] this is true of face shape,[22] but surprisingly not for voices,[23] even though people think they are associated with a person's sexual orientation.[24] A handful of studies have investigated the question of gaydar from the voice.[25][26][27][28] They have found that people can tell who is gay and straight from their voices, but have mostly focused on men (sometimes terming the vocal difference "gay lisp"). Detailed acoustic analyses have highlighted a number of factors in a person's voice[28] that are used, one of which is the way that gay and straight men pronounce "s" sounds.[25]

Research by William T. L. Cox and his colleagues proposed that "gaydar" is simply an alternate label for using LGBT stereotypes to infer orientation (e.g., inferring that fashionable men are gay).[29] This work points out that the scientific work reviewed above that claims to demonstrate accurate gaydar falls prey to the false positive paradox (see also the base rate fallacy), because the alleged accuracy discounts the very low base rate of LGBT people in real populations, resulting in a scenario where the "accuracy" reported above in lab studies translates to high levels of inaccuracy in the real world.

In the early 2000s, an electronic device based on the Japanese Lovegety wireless dating device was marketed as 'Gaydar' and reported on widely in the media.[30][31] This was a key-chain sized device which would send out a wireless signal, alerting the user via a vibration, beep or flash when a similar device was within 12 m (40 ft). This let the user know that a like-minded person was nearby.

In 2017, researchers have claimed that an artificial intelligence (AI) algorithm could correctly identify sexual orientation in 81% of the tested cases for men and 74% with women, (91% and 83%, respectively, when using five images of each subject) just by reviewing a handful of photos of online dating profiles.[32]
In early 2018, other researchers, among which two specialists of AI working at Google (one of the two on face recognition), issued a reportedly contradicting study based on a survey of 8,000 Americans using Amazon’s Mechanical Turk crowdsourcing platform. The survey yielded many traits helping to discriminate between gay and straight respondents with a series of yes/no questions. These traits had actually less to do with morphology than with grooming, presentation, and lifestyle (makeup, facial hair, glasses, selfie angle, etc.)[33]